No space left on device on xfs filesystem with 2.4T

情况说明：日前，同事反馈一个问题：在一个大分区(24T)中使用xfs文件系统，用来做历史文件备份，突然提示没有磁盘空间错误，先检查下：

[root@imysql ~]# df -hT
Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/sdb1      xfs     19T   16T  2.4T  88% /backup
[root@imysql ~]# df -hi
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/sdb1               9.3G    3.4M    9.3G    1% /backup

可以看到，不管是物理空间，还是inode，都还有很多余量，那为何还会报告磁盘空间不够呢？查询了下XFS FAQ，发现有一段：

Q: What is the inode64 mount option for?By default, with 32bit inodes, XFS places inodes only in the first 1TB of a disk. If you have a disk with 100TB, all inodes will be stuck in the first TB. This can lead to strange things like "disk full" when you still have plenty space free, but there's no more place in the first TB to create a new inode. Also, performance sucks.To come around this, use the inode64 mount options for filesystems >1TB. Inodes will then be placed in the location where their data is, minimizing disk seeks.Beware that some old programs might have problems reading 64bit inodes, especially over NFS. Your editor used inode64 for over a year with recent (openSUSE 11.1 and higher) distributions using NFS and Samba without any corruptions, so that might be a recent enough distro.
大意就是xfs文件系统会把inode存储在磁盘最开始的这1T空间里，如果这部分空间被完全填满了，那么就会出现磁盘空间不足的错误提示了。查看mount的man帮助手册，可以看到如下内容：inode64Indicates that XFS is allowed to create inodes at any location in the filesystem, including those which will result in inode numbers occu-pying more than 32 bits of significance. This is provided for back-wards compatibility, but causes problems for backup applications thatcannot handle large inode numbers.
解决办法就是在挂载时，指定inode64选项：

mount -o remount -o noatime,nodiratime,inode64,nobarrier /dev/sdb1 /backup

参考资料：https://www.centos.org/forums/viewtopic.php?t=21766P.S:磁盘空间小于1T的不用担心这个问题.

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